In the pneumatic tires especially that for high performance cars such as sport car, the aspect ratio of the tire is on the decreasing trend.
A conventional carcass structure widely used in such a low aspect ratio tire comprises a carcass ply (b) extending between the bead portions (c) and turned up around bead cores (d) from the axially inside to outside of the tire.
In such a low aspect ratio tire for high performance car, as the tire height is relatively low and the running speed is high, the sidewall portion is required to have a high rigidity to meet severe service conditions.
Therefore, in order to increase the sidewall rigidity, the number of the carcass plies (b) is increased to two plies, and the height of each turned up portion (t) is increased, as shown in FIG. 6.
In such high turnup structure, however, it is difficult to increase the lateral, longitudinal and torsional rigidity of the tire to improve the handling. Further, the total length of the carcass plies is inevitably increased to increase the tire weight.
In order to solve the above-mentioned problems, another carcass structure shown in FIG. 7 has been proposed, wherein the carcass consists of an inner carcass ply (b) tuned up around a bead core (d) from the axially inside to outside of the tire, and an outer carcass ply (f) extending radially inwardly from the tread portion along the axially outside of the inner carcass ply turned up portion (t) and turned axially inside of the tire under the bead core (d). Accordingly, by the inner and outer plies (b and f), a stiff shell structure is formed, and the lateral, longitudinal and torsional rigidity of the tire can be effectively increased.
However, the manufacturing of such a tire is difficult. As a result, not only the production cost is increased, but also the edge (g) of the outer carcass ply (f) under the bead core (d) is easily disturbed, and the force variations of the finished tire are liable to increase to deteriorate the high speed running stability.